The present invention relates generally to a Doppler-based traffic radar system and, more particularly to a radar system and related method for determining the speed of target vehicles independent of the direction of the target vehicles relative to a patrol vehicle supporting the radar system.
Law enforcement officers have utilized Doppler-based traffic radar systems to monitor vehicle speeds and enforce traffic speed limit laws for many years. Throughout this period of time, numerous improvements in both the underlying technology and in the specific application of new processing techniques for the traffic radar systems themselves, have afforded law enforcement officers greater flexibility and improved reliability in carrying out their duties. One such improvement evident in most traffic radar systems presently being marketed includes the capability to more accurately and reliably monitor the speed of certain vehicles while the patrol vehicle is either in a stationary or a moving mode of operation. In fact, present traffic radar systems can now successfully monitor the speed of vehicles approaching the moving patrol vehicle in an opposite lane, and monitor a group of target vehicles simultaneously and to determine the fastest vehicle within the group and/or the vehicle presenting the traditional strongest reflected return signal. Importantly, each of these new or improved capabilities provide the law enforcement officer with a more complete picture of the traffic environment and thus, a more flexible and reliable basis for making more informed decisions.
Despite all the improvements in both the underlying technology utilized in these traffic radar systems and the radar systems themselves, however, a broadly recognized shortcoming of these systems involves the successful monitoring of the speed of vehicles approaching or receding away from the patrol vehicle in a same-lane moving mode of operation or in a stationary mode of operation without operator intervention. Such a radar system and related methods are provided for in the applicant""s U.S. Pat. No. 6,008,752 wherein a direction of a selected target is determined and utilized in calculating its speed both in same-lane moving and stationary modes of operation without the need for operator intervention.
One additional radar system capable of monitoring the speed of vehicles approaching or receding away from the patrol vehicle in a same-lane moving mode of operation is described in U.S. Pat. No. 6,198,427 to Aker et al. A digital signal processor is utilized to perform a complex fast Fourier transform utilizing a first radar return signal as imaginary inputs and a second radar return signal which is shifted in phase by an integer multiple of 90 degrees as real inputs. The output of the complex fast Fourier transform includes an approaching half spectrum of components and a paired receding half spectrum of components as a result of common microwave hardware imperfections. Either or both half spectrums are selectively searched depending upon a mode of operation of the radar system.
One shortcoming of utilizing a complex fast Fourier transform, without more, to obtain target spectrums is the inherent redundancy of targets in the approaching and receding spectrums which necessitate additional qualification testing, such as bleed through directionality testing, in order to prevent false information regarding targets. Even with bleed through testing, common mode signals and mismatches in the microwave hardware may be sufficient to generate additional false target information which are undetectable by bleed through testing and require additional qualification testing to eliminate.
Accordingly, a need is clearly identified for a radar system and related methods capable of simply yet accurately determining the actual speed of target vehicles traveling in the same lane as the moving patrol vehicle, or in a different lane than the stationary patrol vehicle which eliminates the need for bleed through testing and significantly limits the need for qualification testing. Such a radar system and related methods would effectively eliminate the need for bleed through qualification testing, and significantly limit the need for qualification testing caused by common mode signals and mismatches in microwave hardware in determining target speeds.
Accordingly, it is a primary object of the present invention to provide a novel and improved Doppler-based radar system capable of overcoming the limitations of the prior art.
Another object of the present invention is to provide a Doppler-based traffic radar system that increases the overall flexibility and reliability afforded a system operator.
Additional objects, advantages, and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as described herein, a novel and improved Doppler-based traffic radar system is provided that increases the overall flexibility and reliability afforded the system operator, and most importantly, effectively eliminates the need for bleed through testing, and significantly limits the need for qualification testing. More specifically, the radar system of the present invention is adapted to determine the speed of at least one moving target traveling in generally the same direction as a platform supporting the radar system independent of the direction of the at least one target relative to the platform. This allows the speed of target vehicles traveling in the same lane and in the same direction as the patrol vehicle to be monitored without manual intervention by the operator. The radar system is further adapted in a stationary mode of operation to determine the speed of at least one selected moving target independent of the location of the patrol vehicle. This is accomplished in part by selectively eliminating from consideration all unwanted approaching or receding targets depending upon the present traffic/patrol scenario.
The radar system of the present invention splits the return signals reflected from the at least one moving target in order to form a test processing signal and a reference processing signal. More specifically, the return signals received from an antenna are split in a turnstile, thus generating first and second split signals. The split signals are substantially equivalent both containing information about the at least one target. Within the turnstile the first split signal is mixed with a first portion of the transmitted signal to form a test processing signal. Likewise, the second split signal is mixed with a second portion of the transmitted signal to form a reference processing signal.
Any suitable means may be used to insure that the resultant processing signals are different in phase. For example, use of microstrip transmission lines of differing lengths connecting each of a pair of sniffer probes for coupling portions of the transmitted signal to the mixers, and/or transmission lines of differing lengths connecting each of the first and second split signals to the mixers, is sufficient to cause the signals to be different in phase. Alternatively, a phase shifter or other means generally known in the art may be utilized.
Because the resulting processing signals are equal in magnitude, but different in phase, the direction of the at least one target relative to the platform can be determined and the speed accurately calculated. For instance, if the phase of the reference signal for an approaching target leads the phase of the test signal, then the phase of the reference signal for a receding target will lag the phase of the test signal. This phase differential in the processing signals also allows the radar system to more accurately calculate the speed of a selected target in the stationary mode of operation.
In accordance with an important aspect of the present invention, speed determining circuitry, such as a processor or the like, transforms samples of the test and reference processing signals from the time domain to the frequency domain to provide at least one frequency spectrum. As is well known in the art, the resulting frequency spectrum of a time to frequency domain transformation necessarily includes a plurality of bins indicative of the Doppler components of the at least one target. More specifically, the Doppler components include the amplitude and frequency of each target.
The resulting Doppler components of the at least one frequency spectrum are subsequently cross-correlated by the speed determining circuitry to obtain a cross-correlation spectrum of real and imaginary cross-correlated components. In accordance with an important aspect of the present invention, the resulting components of the cross-correlation are indicative of the direction and speed of the at least one target.
During operation, the radar system selects a target from the cross-correlation components according to the present operating mode of the radar system. In a moving, strongest target mode of operation, for example, the cross-correlation components are searched for a target having the highest amplitude. Similarly, in a moving, fastest target mode of operation, the cross-correlation components are searched for a target having the highest frequency. Additional modes of operation of radar systems are generally well known in the art and may require the radar system to determine the speed of a target in a stationary, strongest mode; a stationary, fastest mode; a stationary, strongest and fastest mode; a moving, strongest, opposite lane mode; a moving, strongest, same lane mode; a moving, fastest, opposite lane mode; a moving, fastest and strongest, opposite lane mode; a moving, fastest, same lane mode; a moving, fastest, same lane mode; or a moving fastest and strongest same lane mode.
Once the step of selecting a target is accomplished, the speed of the at least one target is calculated dependent upon a determined direction of the target and displayed by the radar system for the operator""s use. Advantageously, this allows for the speed of multiple targets to be determined and displayed.
Still other objects of the present invention will become readily apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of one of the modes best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments, and its several details are capable of modifications in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.